Combinatorics of minuscule representations BOOK REVIEW

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Combinatorics of minuscule representations BOOK REVIEW

Bulletin of the London Mathematical Society Advance Access published January 22, 2015
Bull. London Math. Soc. Page 1 of 5
BOOK REVIEW
Combinatorics of minuscule representations
(Cambridge Tracts in Mathematics 199)
By R. M. Green: 320 pp., £50.00 (US$80.00), isbn 9781107026247
(Cambridge University Press, Cambridge, 2013).
e 2015 London Mathematical Society
C
doi:10.1112/blms/bdu112
To be a minuscule representation of a complex simple Lie algebra g is to be ‘as small as
possible’: not only irreducible and ﬁnite-dimensional, but with all weights in the same orbit
under the action of the Weyl group W . In particular, the highest weight λ has one-dimensional
weight space, so all of its weight spaces are one-dimensional. It turns out to be fruitful to
partially order the set of weights W λ, deﬁning the weight poset in which μ ν when ν − μ is
a non-negative sum of positive roots. As explained in R. M. Green’s book under review here,
poset structures are the key to a minuscule representation’s many special properties, making
it simpler than a typical g-irreducible.
Minuscule representations exist only in types A, B, C, D, E6 , E7 . In type A, they are the
exterior powers ∧k Cn of the natural representation Cn for the Lie algebra g := sln (C) of
trace zero matrices in Cn×n . Here the Weyl group W of type An−1 can be taken to be
the symmetric group Sn permuting the standard basis e1 , . . . , en . The monomial wedges
{ei1 ∧ · · · ∧ eik }1i1 <···<ik n give a basis of weight vectors for ∧k Cn , depicted here for ∧2 C5 ,
ordered as in the weight poset:
e1 ∧ e2 Q
e1 ∧ e3 Q
m
e2 ∧ e3 Q
e1 ∧ e4 Q
m
e2 ∧ e4 Q
e1 ∧ e5
m
m
e3 ∧ e4 Q
e2 ∧ e5
m
e3 ∧ e5
m
e4 ∧ e5
(1)
It is useful to reinterpret the transitive W -action on W λ as the action on cosets W/WS−s ,
where WS−s is the maximal parabolic subgroup of W ﬁxing λ; a minuscule highest weight λ
is always one of the fundamental weights, ﬁxed by all but one simple reﬂection s among the
Coxeter generators S for W . Stembridge [8] showed that the set W S−s of minimum-length
coset representatives for W/WS−s enjoys two extra properties in the minuscule setting.
(i) Each w in W S−s is fully commutative: its minimum-length factorizations in S (reduced
S-words) can be connected by sequences of commutation moves si sj = sj si valid in W , with
no need for longer braid relations from W .
(ii) The elements w in W S−s are characterized as those with a reduced word which is a
suﬃx of a reduced word for the unique element w0S−s in W S−s that carries the highest weight
to the lowest weight.
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BOOK REVIEW
From this, one can show that the weight poset on W λ is (the opposite of) either the (strong)
Bruhat order or (left) weak Bruhat order restricted from W to W S−s .
For example, when g = sl2 (C) as above, one can take the adjacent transpositions si = (i, i +
1) as the simple reﬂections S = {s1 , . . . , sn−1 }. When n = 5, the minuscule representation ∧2 C5
has its highest weight vector vλ with weight λ ﬁxed by {s1 , s3 , s4 } ⊂ S, that is, by S − s for
s = s2 . The elements of W S−s are the permutations w = (w1 , w2 , w3 , w4 , w5 ) having w1 < w2
and w3 < w4 < w5 . Below is the same weight poset as in (1), depicted as the opposite of the
Bruhat order on W S−s , with w in W S−s indicated by its set of reduced S-words w = si1
si2 · · · si .
∅ WWWWW
WWWWW
WWWWW
WWWWW
WWWWW
WW
gg s2 TTTTT
g
g
g
TTTT
gg
g
g
g
g
TTTT
ggg
g
g
g
TTTT
g
ggg
g
TT
g
g
g
g
s1 s2 WWW
j s3 s2 MM
WWWWW
MMM
jjjj
WWWWW
j
j
j
MMM
WWWWW
jj
j
j
j
WWWWW
MM
jjjj
s1 s3 s2 = s3Ss1 s2
s4 s3 s2
h
SSS
tt
hhhh
SSS
h
t
h
t
h
SSS
h
tt
SSS
hhhh
tt
S
hhhh
t
h
h
h
hh
s1 s4 s3 s2 = s4 s1 s3 s2
s2 s1 s3 s2 = s2 sS3 s1 s2
= s4 s3 s1 s2
SSSS
o
SSSS
o
o
SSSS
ooo
SSSS
ooo
S
ooo
s2 s1 s4 s3 s2 = s2 s4 s1 s3 s2
= s4 s2 s1 s3 s2 = s2 s4 s3 s1 s2
= s4 s2 s3 s1 s2
lll
l
l
ll
lll
lll
w0S−s = s3 s2 s1 s4 s3 s2 = s3 s2 s4 s1 s3 s2
= s3 s4 s2 s1 s3 s2 = s3 s2 s4 s3 s1 s2
= s3 s4 s2 s3 s1 s2
Its bottom element is the unique permutation w0S−s = (4, 5, 1, 2, 3) in W S−s carrying the
highest weight e1 ∧ e2 to the lowest weight e4 ∧ e5 , and every w in W S−s has an S-reduced
word that is a suﬃx for one of the S-reduced words of w0S−s .
This weight poset W λ, or Bruhat order on W S−s , is far better behaved than your average
poset. It is a lattice, meaning that pairs of elements have a well-deﬁned join (least upper bound)
and meet (greatest lower bound). Better yet, it is distributive: the meet and join operations
distribute over each other. A theorem of Birkhoﬀ (see, for example, [6, Theorem 3.4.1]) then
implies that it is determined in a simple way by its subposet E of meet-irreducibles (the
poset elements covered by only one other element): the distributive lattice is isomorphic to
the opposite of the inclusion poset J(E) on the collection of order ﬁlters (subsets closed under
going upward) in E. Furthermore, the above-mentioned fully commutative and suﬃx properties
let one identify E with the minuscule heap associated to λ and w0S−s , which projects vertically
BOOK REVIEW
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to the Dynkin diagram Γ, as shown here:
s2
 ???


s3
s1 =
=== ====
s4
s2 =
=== s3
s1
heap poset E
(2)
s2
s3
s4
Dynkin diagram Γ
The linear extensions of the heap poset E are the S-reduced words for w0S−s . More generally,
the isomorphism alluded to above sends an order ﬁlter of the heap poset to the element w in
W S−s factored by its set of linear extensions. For example, it maps this order ﬁlter as follows:
s2 ?
 ???

?

s1 ?
s3
??
 ???
?? 
??

s2
s4
/
w = s2 s1 s4 s3 s2 = s2 s4 s1 s3 s2 = s4 s2 s1 s3 s2
= s2 s4 s3 s1 s2 = s4 s2 s3 s1 s2
Note in (2) that the map : E → Γ from the heap poset E to the Dynkin diagram Γ is just the
labeling of poset elements by simple reﬂections. Additionally, each node si and edge {si , sj }
of the Dynkin diagram Γ have inverse images −1 (si ), −1 ({si , sj }) which are totally ordered
(chains) in E, and these chains characterize the poset E as the weakest partial order extending
all of these chain orders.
The latter axiomatic properties of heaps over Dynkin diagrams are the starting point for
Green’s book. After the quick introductory Chapter 1 on classical Lie algebras and Weyl groups,
Chapter 2 dives into the seemingly innocuous theory of heaps over graphs and over Dynkin
diagrams, and their order ideals (sets closed under going downward, the complements of order
ﬁlters). It introduces the crucial notion of a full heap over a Dynkin diagram. Chapter 3 shows
how the Weyl group W acts on the subset of proper order ideals of any full heap over its
Dynkin diagram.
Chapters 4–7 are the technical heart of the book. They review more Lie algebra theory,
Weyl groups, Chevalley bases, and eventually show how minuscule representations come from
heaps. The upshot comes in § 6.6, asserting that every minuscule representation of a simple
Lie algebra g over C can be constructed from a principal subheap of a full heap over an aﬃne
Dynkin diagram.
Remark. Here ‘technical’ does not mean hard, but that at times it requires case-by-case
checks, folding arguments for non-simply laced cases, and can get a bit grungy. In particular,
§ 7.4 is notable for a long sequence of lemmas beginning either with ‘Maintain the notation of
Deﬁnition. . . ’ or ‘Maintain the hypotheses of Proposition. . . ’.
Chapter 8 is a fascinating excursion into properties of W acting on W λ, including
(i) the poset isomorphism between W λ and the subposet of positive roots lying above the
unique simple root α non-orthogonal to λ;
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BOOK REVIEW
(ii) a description of the W -orbits on W λ × W λ and its relation to branching rules for
minuscule representations; and
(iii) structural results on the weight polytope which is the convex hull of W λ.
In Chapters 9 and 10, the author really warms to his subject. It is a detailed exposition of the
relation between exceptional minuscule representations (two of dimension 27 in type E6 , one of
dimension 56 = 2 · 28 in type E7 ) and the classical incidence geometry of special conﬁgurations:
the 27 lines on a smooth cubic surface, the 28 pairs of lines on a degree 2 del Pezzo surface
or bitangents to a non-singular plane quartic curve, Schläﬂi’s double sixes, Steiner complexes,
generalized quadrangles, etc. The discussion relies on algebro–geometric results of du Val and
the recent work of Dolgachev [1] to get the ball rolling, then proceeds into incredible detail.
As a neophyte in these topics, I was overwhelmed, but impressed.
The ﬁnal Chapter 11 returns to brieﬂy treat more standard topics, mainly related to the
enumerative combinatorics of minuscule heaps as posets, such as the product formula for their
rank generating function. Most proofs are omitted in this chapter. It is an old story, well-told by
Proctor [3], and Stembridge [7] (see also Stanley’s exercises related to pleasant and Gaussian
posets in [6, Chapter 3, Exercises 170, 172]) how one can deduce these results by combining
the q-analog of Weyl’s dimension formula with Seshadri’s standard monomial theory for the
coordinate rings of the minuscule ﬂag variety G/PS−s . Here PS−s is the parabolic subgroup
associated to WS−s inside the complex reductive Lie group G associated to g. Chapter 11
also discusses Stembridge’s application of minuscule posets to his q = −1 phenomenon for
self-complementary plane partitions [7], along with very recent work of Rush and Shi on the
cyclic sieving phenomenon for the Fon-Der-Flaass or rowmotion action on minuscule posets
[5]. Finally, it discusses recent results of Thomas and Yong [9], applying Proctor’s theory [4]
of jeu-de-taquin on the minuscule heap to do Schubert calculus for the minuscule ﬂag varieties
G/PS−s . In spite of its brevity and lack of proofs, I found Chapter 11 to be an excellent
collection of my favorite minuscule topics, and a very useful update to the older surveys such
as Proctor [3] or Hiller [2, Chapter 5].
I like Green’s book. The Introduction sets out the plan well, the end notes for each chapter
are helpful, the bibliography is thorough and up-to-date, and it is the most thorough source for
the heap-theoretic approach. It has many good exercises, although I might hesitate to use it as
a course text, for fear of exhausting some students. I did have a few minor terminology quibbles,
listed in the Remark below. Nevertheless, I think it will serve as a very useful updated reference
for combinatorialists and Lie theorists who use minuscule representations in their work.
Remark. In § 2.1, the computer science terminology ‘trace’ for elements of commutation
monoids conﬂicts with trace of matrices. The notation Tp,q in § 3.1 confused me, as q is a Hecke
algebra parameter, while p is a Dynkin diagram node; it looks like q, p should play symmetric
roles. The terminology ‘complemented’ in § 11.2 for posets with an order-reversing involution
is non-standard.
References
1. I. V. Dolgachev, Classical algebraic geometry: a modern view (Cambridge University Press, Cambridge,
2012).
2. H. Hiller, Geometry of Coxeter groups, Research Notes in Mathematics 54 (Pitman (Advanced Publishing
Program), Boston, MA, 1982).
3. R. A. Proctor, ‘Bruhat lattices, plane partition generating functions, and minuscule representations’,
European J. Combin. 5 (1984) 331–350.
4. R. A. Proctor, ‘d-Complete Posets Generalize Young Diagrams for the Jeu de Taquin Property’, Preprint,
2009, arXiv:0905.3716.
5. D. B. Rush and X. Shi, ‘On orbits of order ideals of minuscule posets’, J. Algebraic Combin. 37 (2013)
545–569.
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Page 5 of 5
6. R. P. Stanley, Enumerative combinatorics, vol. 1, 2nd edn, Cambridge Studies in Advanced Mathematics
49 (Cambridge University Press, Cambridge, 2012).
7. J. R. Stembridge, ‘On minuscule representations, plane partitions and involutions in complex Lie groups’,
Duke Math. J. 73 (1994) 469–490.
8. J. R. Stembridge, ‘Minuscule elements of Weyl groups’, J. Algebra 235 (2001) 722–743.
9. H. Thomas and A. Yong, ‘A combinatorial rule for (co)minuscule Schubert calculus’, Adv. Math. 222
(2009) 596–620.
Victor Reiner
School of Mathematics
University of Minnesota
Minneapolis, MN 55455
USA
[email protected]·umn·edu